1. Field
This invention relates to NMR imaging (Magnetic Resonance Imaging, MRI) systems and techniques, and more particularly to an NMR rapid line scan technique for developing two-dimensional images from a plurality of line scans.
2. State of the Art
Nuclear magnetic resonance (NMR) is presently a recognized phenomenon in which the nucleus of selected atoms can be excited by a particular or selected resonant radio frequency (RF) signal. Based on this phenomenon, a number of NMR techniques have been devised to generate images of a specimen. NMR apparatus employing these techniques are capable of producing complicated pulse sequences automatically and are capable of automatically recording certain echo signals resulting from those pulse sequences as applied to a particular target or specimen for further processing through a compute to produce the images of the specimen which may be regarded as a slice image.
NMR imaging apparatus are described in the book, NMR Imaging in Biomedicine, P. Mansfield and P. G. Morris, 1982 Academic Press, New York, N.Y. (Library of Congress Catalog Card No. 65-26774). NMR imaging apparatus and techniques are also described in U.S. Pat. No. 4,599,565 (Hoenninger, III et al.); U.S. Pat. No. 4,614,195 (Bottomley, et al.); and U.S. Pat. No. 4,583,044 (Case et al.).
The NMR imaging described in the above-noted patents and text is based on the ability to induce and monitor the resonance of the magnetic moment of selected nuclei in the presence of magnetic fields. If the magnetic fields are varied spatially, it is possible to measure both the location and concentration in small volumes of particular nuclei and to process that data with a computer into a visual display or slice image that reflects the distribution of those nuclei in the tissue or the sample. In medical applications, NMR imaging is a noninvasive diagnostic technique which has some general similarity to computer tomography scanning (utilizing X-ray radiation), even though it is based upon an entirely different physical phenomenon.
NMR systems typically include a large magnet to create a homogenous magnetic field around and within the target or specimen as well as a plurality of coils to create position-dependent or spatially-related fields within the homogenous field. Further, one or more RF coils are positioned near or around the target or specimen to apply the resonant radio frequency (RF) signals and receive the NMR signal. Associated electronic circuitry processes the received signals (called spin echoes) into information suitable for display on a screen such as a CRT tube.
U.S. Pat. No. 4,599,565 (Hoenninger, III et al.), describes a two-dimensional Fourier transform (2DFT) method. Various spaced apart subvolume regions of a particular sample are each sequentially excited in order to allow sufficient time for the excited nuclei or atomic particles in each subvolume to return to normal or to relax. In U.S. Pat. No. 4,599,565, spatial division multiplexing techniques are used to develop an acceptable image over time. However, the technique needed to develop the image permits artifacts due to movement of the specimen.
In typical imaging techniques including NMR techniques heretofore known, sufficient time elapses between the beginning and the completion of the sampling so that any motion of the object being sampled or examined results in substantial artifacts or ambiguities in the reproduced image. For example, if 2DFT NMR imaging techniques are used to generate an image of the chest area proximate the heart, the motion of the heart as it beats is sufficient to generate substantial artifacts or ambiguities in the signal. Similarly, motion of the chest wall from breathing may cause substantial artifacts or ambiguities, and thereby prevent the use of NMR imaging as an effective medical diagnostic tool.
There is a need for an NMR imaging system and method in which the motion of the object does not produce artifacts or ambiguities in the reproduced image constructed and displayed on an appropriate video screen.